1. Field of the Invention
The present invention relates to resistance welding apparatus, particularly to an equalizing mechanism which allows the welding apparatus to follow a workpiece.
2. Description of the Prior Art
Resistance welding is to locally heat a workpiece held together between two electrodes by intensively applying electric current flow and pressure thereto with the electrodes.
Such welding apparatus is connected to a robot arm or placed in a predetermined position to perform the weld on an accurate position of workpiece with a robot teaching system. Equal electrode pressure is applied on both sides of the workpiece when the workpiece and the electrodes are accurately positioned. However, the electrodes tend to wear, and it is very hard to constantly maintain the positions of the workpiece and the electrodes. Moreover, with use of a robot having a low teaching accuracy or workpiece pressed with a low precision, the workpiece and the electrodes are likely to be inaccurately positioned, causing such a problem that the workpiece is deformed by a pressurization of the welding apparatus.
With a view to solving this problem, the welding apparatus needs to include an equalizing mechanism for negating an error in the positional relation between the workpiece and electrodes.
Japanese Patent Application Publication No. 2004-090020 discloses conventional welding apparatus and equalizing mechanism, for example. With reference to FIG. 9, welding apparatus 110 comprises a fixed bracket 120 connected to a robot arm and an apparatus body 130 to be equalized. The apparatus body 130 comprises an arm 132 including a pressure shaft 151 on one end on which one electrode 181 is mounted and an other electrode 182 is mounted on the other end of the arm 132.
The pressure shaft 151 is driven by a motor 159 to move forward and backward and the electrode 182 is placed on the axial line. Thus, the two electrodes 181, 182 are configured to relatively approach or move away from each other on a substantially straight line.
An equalizing mechanism 190 is provided to connect the fixed bracket 120 and the apparatus body 130, and is driven by a motor 199 to move the apparatus body 130 relative to the fixed bracket 120 in parallel to the pressure shaft 151. The two motors 159, 199 are servo motors having encoders thereon to be able to accurately control the positions of the electrodes 181, 182 using positional data from the encoders, torque of the motors 159, 199, and current flow data.
However, there is a problem with this welding apparatus 110 in an increase of the size due to use of the two motors 159, 199, and difficulty in compact design. Further, controlling the two motors 159, 199 along with the robot arm requires a complicated control system, which needs a large amount of time and labor to set the system.